LCDs, such as multiplexed Twisted Nematic TN-LCDs, Super Twisted Nematic STN-LCDs, Super Birefringent SBE-LCDs, Electrically Controlled Birefringence ECB-LCDs or flexoelectric LCDs are currently used or being developed for computer monitors, laptop or notebook computers, portable telephones, personal digital assistants, etc. The optical, electrical and temporal performance, e.g., contrast, threshold and driving voltages, and response times, of such displays depends crucially on the ratios of the elastic constants (k.sub.33, k.sub.22, k.sub.11). Currently commercially available nematic mixtures for sophisticated high-information-content LDCs, such as STN-LCDs, generally incorporate trans-1,4-disubstituted-cyclohexyl derivatives with a terminal alkenyl chain (i.e., incorporating a carbon--carbon double bond) directly attached to the cyclohexane ring in order to produce the necessary elastic constant ratios for short response times, high multiplexing rates and low driving voltages. Such materials are costly and difficult to synthesise due to the requirement for a trans configuration of the 1,4-disubstituted cyclohexane ring and the necessity of synthesising the carbon--carbon double bond stepwise from this trans-1,4-disubstituted-cyclohexyl intermediate. If the carbon--carbon double bond is substituted at both carbon atoms, it must have a trans (E) configuration in order to exhibit an advantageous combination of elastic constants and to have an acceptably high nematic-isotropic transition temperature (N-1). The trans configuration is then generally produced by isomerisation of the cis (Z) form generated by the preceding Wittig reaction.
Light crystalline bicyclo[2.2.2]octanes are known and are described in for example G W Gray et al, J. Chem. Soc. Perkin II, (1981), pp 26-31, N Carr et al (1981), Vol 66, pp 267-282, (1985), Vol 129, pp 301-313, R Dabrowski, Mol. Cryst. Liq. Cryst., (1991), Vol 209, pp 201-211, R Dabrowski, Ferroelectrics., (1991), Vol 114, pp 229-240.
For all the above applications it is not usual for a single compound to exhibit all of the properties highlighted, normally mixtures of compounds are used which when mixed together induce the desired phases and required properties.